The search has also moved on to the Glen Head area and a Coastguard helicopter from Strandhill, Co. Sligo, has joined the operation along with units from Killybegs Coastguard and three teams from Donegal Mountain Rescue.

As darkness fell last night, teams assisted by volunteers checked outbuildings and ditches in the area.

David Murray, of Donegal Mountain Rescue, said: 'If she has been out for two nights there is a strong possibility of hypothermia.

'It is a difficult terrain. There are a lot of crags, a lot of rock. There are also some bog-holes.'

Beautiful but rugged: Miss Kelly disappeared for four days while hillwalking in Donegal

'We are right up against the clock,' he said.

Ms Jones is originally from Texas, but has been working in the catering industry in North Carolina.

She is 5ft 6ins and was last seen wearing a green jacket, black trousers and red/maroon hat.

The U.S. embassy in Dublin has been contacted and her father is preparing to fly to Ireland to assist with the search.

He gave permission for Ms Jones's
passport photograph to be released in the hope someone may have seen her
since the last sighting at 5pm on Saturday.